The present invention relates to a bias cleaning system, forming a cleaning unit for removing adhered toner from a member carrying toner, such as a transfer belt, an intermediate transfer drum and so on, and to an electro-static printing apparatus therewith and an operating method thereof.
A brush cleaner, a blade cleaner or a bias cleaning system is used as a cleaning system for removing remaining toner on a photosensitive body.
A typical bias cleaning system comprises an electrically conductive brush roller and a collecting roller. The process to remove toner on the surface of a photosensitive body involves applying a voltage to the electrically conductive brush roller and the collecting roller, scrubbing the surface of the photosensitive body with the electrically conductive brush roller, so that the toner on the surface of the photosensitive body electrically and mechanically adheres to the electrically conductive brush roller, and transferring the toner from the electrically conductive brush roller to the collecting roller, which is supplied with higher voltage than the electrically conductive brush roller. The basic construction of a bias cleaning system of this type is described in, for example, Japanese Patent Publication No. 56-40349 (1981).
Further, a cleaning system having means for aligning the polarity of charged toner on a photosensitive body is described in Japanese Patent Application Laid-Open No. 63-15278 (1988). The system will be explained below, referring to FIG. 28 and FIG. 29. The explanation will be made on the assumption that the polarity of the charged toner is negative.
As shown in FIG. 28, the reverse side surface of print paper 7 is positively charged using a transfer corotron 11 so as to have an opposite polarity to that of the toner 8a (negatively charged). Therewith, the toner 8a developed on a photosensitive drum 1 is transferred to the print paper 7. However, since the positive charge generated by a peeling discharge is supplied to the side of the photosensitive drum 1 charged negatively when the print paper 7 is peeled off from the photosensitive drum 1, a part of the toner 8b remaining on the photosensitive drum 1 after the transferring operation is changed to the opposite polarity due to its receiving a positive charge.
In a case where the print paper 7 is not transported to the transfer portion due to a paper jam or the like, the toner on the photosensitive drum 1 virtually becomes opposite in polarity (positive polarity) because of its receiving a positive charge directly from the transfer corotron 11. The remaining toner on the photosensitive drum 1 cannot be cleaned with a bias cleaning system. Therefore, as shown in FIG. 29, the charge polarity of the toner charged to the opposite polarity (positive polarity) is aligned in negative polarity by providing a pre-charger 28 generating a charge having a polarity (negative polarity) opposite to the voltage applied to electrically conductive brush roller 17.
A bias cleaning system has been used as a cleaning system for a photosensitive body, but has not been employed as a cleaning system for cleaning toner adhering to a toner carrying member other than a photosensitive body, such as a transfer belt, transfer roller, intermediate transfer body or the like.
In an electro-static printing apparatus using a transfer belt, a print paper sheet is electro-statically tacked to the transfer belt made of a dielectric material, such as urethane rubber, so as to be transported to a transfer position. In the transfer position, the toner image on the photosensitive drum having a negative charge is transferred to the print paper by the action of a positive charge applied on the reverse side surface of the transfer belt by a corona charger. Therein, in a case where the print paper does not arrive at the transfer position due to a paper jam or where the print paper arrives and is passed through the transfer position in a partly folded state, the toner image on the photosensitive body is directly transferred to the surface of the transfer belt.
The reverse side surface of a following sheet of paper is spoiled unless the toner is completely cleaned. Further, since the tacking force between the print paper and the transfer belt is decreased, there arises a phenomenon in which the print paper wraps around the photosensitive drum after transferring the toner image (drum wrap). Therefore, it is absolutely necessary to improve the performance of the cleaning system for a transfer belt.
Incidentally, the different points of cleaning for a transfer belt from cleaning for a photosensitive body are as follows:
(1) the toner on a transfer belt is uniformly aligned with a negative polarity since the toner is a transferred toner, and
(2) the positive charge given by a corona charger remains on the reverse side surface of the transfer belt.
Therefore, for cleaning a transfer belt, there is no need to provide means for aligning the charge polarity upstream of the bias cleaning system, as described in Japanese Patent Application Laid-Open No. 63-15278 (1988). On the other hand, when a negative charge applying means is provided, the adhesive force of the toner to the transfer belt becomes large, since the charged amount of the toner negatively increases significantly, which causes a decrease in the cleaning ability. Thus, the conventional approach is not suitable for a bias cleaning system for a transfer belt.